A typical portable electronic device, such as a notebook personal computer, is operated by a battery attached to the device and by an external power supply unit such as an AC adapter. The battery used for the portable electronic device is charged with a charging current supplied from the external power supply unit connected to the device. A large inrush current may flow through the battery when the external power supply unit is connected to the device or when the battery is attached to the device. The electronic device is required to prevent such an inrush current from flowing through the battery.
Some portable electronic devices conventionally use rechargeable batteries as their drive power supplies. Such a conventional electronic device to which a rechargeable battery is attached includes a charging circuit for generating charging current that charges the rechargeable battery with an external power supply unit (refer, for example, to Japanese Patent No. 3428955).
A charging circuit 11 arranged in a conventional electronic device will now be described with reference to FIG. 1.
The charging circuit 11 receives direct-current adapter voltage VAC from an AC adapter 12 connected to the electronic device. The charging circuit 11, which is a DC/DC converter, converts the adapter voltage VAC into output voltage Vout. Error amplifiers 13 to 16 generate a control current Isc in accordance with an output current Iout flowing through a resistor R1, a charging current Ichg flowing through a resistor R2 connected to a battery BT, and a terminal voltage of the battery BT. A pulse width modulator (PWM) 17 changes the duty cycle for activating and inactivating metal oxide semiconductor (MOS) transistors T1 and T2 in accordance with the control current Isc. The charging circuit 11 supplies output power, in accordance with the duty cycle, to a system circuit 19 via a system DC/DC converter 18. At the same time, the charging circuit 11 charges the battery BT.